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Scanning Electron Microscope

 
 
 
 
 

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电子通道反差图像技术  

2012-01-13 14:32:38|  分类: 默认分类 |  标签: |举报 |字号 订阅

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Introduction

Most of engineering metallic materials are polycrystals. The individual grain usually contains several kinds of lattice defects like dislocations. The dislocations play an important role in strained crystals. When the applied stress exceeds the yield point, the crystal begins to deform plastically by dislocation movement along slip planes. With increasing plastic deformation, the dislocation density would increase due to the occurence of multiplication mechanisms. Such an increase in dislocation density causes strain hardening of crystals.


During fatigue deformation frequently occurs to and fro motion and cross slip of dislocations. Such dislocation motions induce multiplication and mutual annihilation of dislocations. If these multiplication and annihilation are repeated suffciently, the dislocations are usually self-organized into dislocation bundle structure called 'vein'. The vein structure is composed from high and low dislocation density regions which have irregular shapes. Further fatigue cycling can give rise to the formation of 'persistent slip bands (PSBs)' along the primary slip plane in the vein structure if plastic strain amplitude is in a certain range. The PSBs were observed first by optical microscope observations as a slip band persistently reappeared by load cycling even after specimen surface layer was removed. The dislocation structure of the PSB has been studied using transmission electron microscopy (TEM). The morphology of the PSB dislocation structure is characterized by ladder-like dislocation structure as schematically shown in the figure beside.
Schematische Dastellung der PSBs
Source: Kaneko, Hashimoto (2003)

It has been recognized that intragranular fatigue cracks are nucleated prefentially at PSBs. Stress concentration due to the rough surface is expected to cause the preferential crack nucleation. During further fatigue loading, the nucleated cracks continue to propagate, and subsequently a final fatigue rupture occurs. It is well known that such fatigue rupture often lead to serious accidents in various engineering structures. In order to prevent the fatigue rupture, nondestructive inspections including X-ray and ultrasonic wave methods have been employed for detecting fatigue cracks. However, in order to improve the prevention of the fatigue rupture, it is desirable to perform prediction of fatigue crack nucleation rather than the detection of the cracks which are already nucleated. Because the PSB formation precedes to the fatigue crack nucleation it is thinkable that the detection of the PSb formation corresponds to the prediction of the fatigue crack nucleation. However, the conventional TEM observation must include a shaping process to a thin foil. So the TEM observation is inadequate for the inspection of engineering parts which should be reused for further observations. Accordingly, a nondestructive method to observe dislocation structures is desirable instead of the TEM.


The ECCI technique

Recently, a new observation technique called 'electron channeling contrast imaging (ECCI) has been employed to image dislocation structures. The ECCI technique has a characteristic feature that the dislocation lying close to the crystal surface can be detected nondestructively using scanning electron microscope (SEM). In addition the ECCI has some advantages in examining the dislocation structure in the fatigued materials. If we use the TEM, visible field of a thin foil sample is limited to narrow area where the incident electron beam can pass through. Hence, the TEM observation at low magnification such as x100 is almost impossible. In such a situation, there is a risk that one can miss the the PSB formation if the total number of the PSBs generated in material is very low. Moreover, total surface layer of materials is difficult to be observed by the TEM because of difficulty in foil preparation. This seems inconvenient for investigations of fatigue processes since most fatigue cracks are nucleated at the surface layer. On the other hand, the ECCI technique enables us to observe the dislocation structure near the surface at various magnifications. Specimen preparation is significantly easier to handle than in TEM. Electrolytic polishing satisfies to remove topography of the surface.

Scheme of the effect of a defect on the BSE intensity
Source: Ahmed, Wilkinson, Roberts (1999)

For ECCI technique, the SEM operates at high magnifications and low working distance. Thus, the beam tilting above the imaged specimen region is negligible. Visible contrast is caused by differences in orientation of individual grains or by local tilting of the lattice planes as a result of a accumulation of lattice defects. If the crystal is oriented in Bragg condition local bending of the crystallographic planes where dislocations emerge at the specimen surface cause the required contrast. Near a dislocation a modulation of the BSE intensity is received as shown in the figure beside. Because of the interaction of the electron beam with the specimen the information depth into the crystal lattice is only upt to 100 nm. Hence, surface damages and oxide layers can influence the image quality. Therefore this has to be avoided by specimen preparation.

ECCI observations provide diffraction contrast effects similar to those in TEM. To display contrast in SEM corresponding to bright field images im TEM (dark dislocations on a bright background) the SEM can operate in inverted image contrast.





Results

PSBs in polykristallinem Ni - 1 Ni polycrystals fatigued at constant plastic strain amplitude for 20000 cycles. Subsequently a electolytic polishing was carried out to obtain a plane surface free from stress and topography. The dislocation structure generated by fatigue can be seen in the figure beside. This image is not inverted i.e. the region with high dislocation density are displayed bright and the regions with low dislocation density are displayed dark. Narrow and broad PSBs running from down left to top right can be seen. Around the ladder structure the bundle structure of the matrix is observable.



PSBs in polykristallinem Ni - 2 In this figure a section of the figure above is shown at higher magnification. Contrast is inverted, therefore the image is equivalent to a bright field image in TEM. Regions with high dislocation density are displayed dark and regions with low dislocation density are displayed bright.



PSBs in polykristallinem Ni - 3 Magnificated view of the narrow ladder from the figures above in regular image contrast of ECCI (not inverted). The width of the PSB is less than 1 μm.



PSBs in polykristallinem Ni - 4 Magnificated view of the broad ladder from the figures above in regular image contrast of ECCI (not inverted). The width of the PSB is less than 5 μm.





Conclusion and outlook


By means of ECCI technique dislocation structures can be imaged in SEM without any additional equipment and with less efforts than in TEM. Observation in SEM can result from bulk specimen and from large specimen regions. Specimen is little affected by preparation, therefore the same specimen can be observed after several numbers of cycles and consequently can provide information about the evolution of dislocation structures e.g. the PSBs. In-situ test can be done as well, which is unthinkable with thin foils in TEM. An additional application is the observation of transcrystalline cracks which are nucleated at PSBs whereas estimation of the plastic zone size is possible as well.


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